Modify __main__.py to allow running from wheel

Remove an unused constant

Documentation for --exists-action option

Fixes #1458

pip docs refactor

Simplify __main__ logic

Cleanup after each test case

Revert "Use Travis to handle the Matrix"

This reverts commit 01894aee.

runner.py seemed to exist for the sole function of containing
a simple function that was called by __main__.py.

I've simply removed that file with its function moved into
__main__.py itself; no functionality should be expected to change.

fixes for downloading wheels (Issue #1112)

'pip wheel' should download wheels, when it finds them

Modify add pip_zip to sys.path method

[WIP] Use Travis to handle the Matrix

More flake8

Use Flake8 to enforce code style